1. Field of the Invention
The present invention relates to safety apparatus for use in the drilling and workover of bore holes in the earth for the exploration and production of minerals or geothermal energy sources. Specifically, the invention relates to control method and apparatus which permit automatic application of hydraulic closing pressure in proportion to the well-bore pressures encountered, without applying excessive closing pressure.
2. Description of the Related Art
A major concern in the drilling and workover of bore holes in the earth is the containment of pressure encountered in the well bore. To prevent expensive and dangerous blowouts of gas and/or liquids, pressure-retaining mechanical devices are mounted at the top of the well-bore casing during normal drilling operations. The "blowout preventers"(B.O.P.'s) are designed to close completely on an open hole, or to close on the outer surface of a tubular member that is used in the drilling or completion of any well bore to mechanically contain the well-bore pressure in the annular space between the well-bore casing and the tubular member.
There are two types of designs for blowout preventers. One is the ram-type, which uses opposing hydraulically-driven rams mounted to move perpendicularly to the axis of the well bore. The rams are fitted with elastomeric gaskets. When actuated laterally toward the well-bore axis, the rams close around and seal to the drill pipe and to the B.O.P. housing. The other type of preventer is referred to as "annular,""spherical," or "bag type." In this design, a rubber element encircles the drill pipe. Hydraulic pressure is applied to the rubber element to force it radially inward until contact with the pipe is made. In both cases, the preventers retain the pressure in the annular space between the drill pipe and the casing.
The annular preventer is necessary for new drilling applications. These include (1) underbalanced horizontal drilling projects, in which the weight of the drilling fluids used in the well bore is not sufficient to contain the down-hole pressure; and (2) the workover of wells containing existing well-bore pressure requiring continued drilling or workover operations after the blowout preventer has closed. In these situations, the operator maintains control of the well by applying hydraulic closing pressure to the annular blowout preventer.
Under prior art, the operator has had to guess at the amount of hydraulic pressure necessary to retain the well-bore pressure. An operator ordinarily tends to overcompensate and apply more hydraulic closing pressure than is actually necessary to maintain control of the well. The excess pressure applied accelerates wear of the blowout preventer element and damages the tubular element closed in the annular preventer.
In addition, under prior art, the annular preventer could not be operated until a detectable amount of gas had been released and was present below the rig floor. Such a situation could have serious consequences if an operator with slow reaction time delayed applying hydraulic pressure to close a well.